Might Mercury be a Chthonian planet?

[SamCashion]

delete me.jpg
I was reminded today of the idea of a chthonian planet. For those of you who may not know (although I'm sure the overwhelming majority already do), a Chthonian planet is a remnant of a gas giant that has been stripped of everything but its core, leaving behind what is more or less a terrestrial planet.
I got to thinking, and wondered whether Mercury might be such an object. I did a quick Google search, but nothing reputable came up. Now I very much doubt that I, being unqualified, could propose an idea that hadn't been seriously proposed and actually turn up being correct. Thing is, I wonder why this question hasn't been posed before. I suspect that there is something quite obvious about the nature of Mercury that means it can not have originated as a chthonian planet that I am not aware of. Please enlighten me.

By the way, mods, I was going to post this in the proving grounds, but then I noticed there was a rule that you mustn't still be developing your idea when you put it there. I've posed this question here, as it seems the only other appropriate place to say this, as I'm not going to be defending my idea, just wondering whether it could be plausible. (Do move it accordingly though, if I was wrong)

[Shaula]

I'd say the issues with that could be:
1) couldn't have formed as a gas giant where it was - too much stellar outflow, too hot.
2) forming further away and migrating in would've an option but would have played hell with the orbits of the other planets as it interacted with them on the way in
3) the iron rich (high density) stuff it is made from is consistent with current models for how it formed (the differentiation of the disk due to temperature)

[SamCashion]

I'd say the issues with that could be:
1) couldn't have formed as a gas giant where it was - too much stellar outflow, too hot.
2) forming further away and migrating in would've an option but would have played hell with the orbits of the other planets as it interacted with them on the way in
3) the iron rich (high density) stuff it is made from is consistent with current models for how it formed (the differentiation of the disk due to temperature)

The latter two are compelling, but your first issue may not hold up. What of all the hot Neptunes and hot Jupiters astronomers have discovered; hadn't they formed just as close or closer to their respective stars as Mercury to the Sun?

[Shaula]

Pretty sure the current models say that they formed further out and migrated in (hence the generally elliptical orbits). Further out than Mercury at least. Even with the Sun in a cool phase I am not sure it has the time to form a gas giant there before outflow and temperature stops it forming. I'd need to go look it up to be sure.

[korjik]

The latter two are compelling, but your first issue may not hold up. What of all the hot Neptunes and hot Jupiters astronomers have discovered; hadn't they formed just as close or closer to their respective stars as Mercury to the Sun?

Current theory says that Hot gas giants migrate inward after formation. As a matter of fact, current theory says that migration should be pretty common. The current theory of our solar system originally had Neptune's inside Uranus' orbit.

[IsaacKuo]

Isn't Mercury too puny to be the core of a gas giant? Doing a quick search suggests minimum core mass of somewhere between .75 to 10 Earth masses.

[SamCashion]

I'd say you three have more than adequately answered my question thanks again, guys!

[antoniseb]

All of the above is not to say that Mercury never had a substantial atmosphere as a protoplanet.

[Trakar]

Isn't Mercury too puny to be the core of a gas giant? Doing a quick search suggests minimum core mass of somewhere between .75 to 10 Earth masses.

This was my first concern as well, along with the structure of our system making it unlikely to have had an outer system migrator. Additionally, at Mercury's orbit you aren't really close enough to the Sun to have cooked off the atmosphere of a gas giant.

[Trakar]

All of the above is not to say that Mercury never had a substantial atmosphere as a protoplanet.

The size of its core suggests that it was a much larger planet at one point, no telling what interesting properties it possessed in that earlier incarnation.

[danscope]

Surely Mercury's proximity to the Sun would make it a candidate for losing any atmospere to a severe coronal ejection as in a direct hit.
It wouldn't be a good day on Mercury.

[WayneFrancis]

delete me.jpg
I was reminded today of the idea of a chthonian planet. For those of you who may not know (although I'm sure the overwhelming majority already do), a Chthonian planet is a remnant of a gas giant that has been stripped of everything but its core, leaving behind what is more or less a terrestrial planet.
I got to thinking, and wondered whether Mercury might be such an object. I did a quick Google search, but nothing reputable came up. Now I very much doubt that I, being unqualified, could propose an idea that hadn't been seriously proposed and actually turn up being correct. Thing is, I wonder why this question hasn't been posed before. I suspect that there is something quite obvious about the nature of Mercury that means it can not have originated as a chthonian planet that I am not aware of. Please enlighten me.

By the way, mods, I was going to post this in the proving grounds, but then I noticed there was a rule that you mustn't still be developing your idea when you put it there. I've posed this question here, as it seems the only other appropriate place to say this, as I'm not going to be defending my idea, just wondering whether it could be plausible. (Do move it accordingly though, if I was wrong)

This topic has come up before and from memory here are some of the why's of it not being a gas giant core.

1) to small, even the smaller ice giants are thought to have a core about the size of Earth and Mercury is only about 5% of the mass of the Earth.

2) It wouldn't have had enough material at that orbit to collect to form a gas giant

3) If it formed further out then its migration in would have to be explained and that is very hard with 3 other planets in the way that didn't get tossed out.

4) I seem to remember that there where other properties that a core would have that mercury doesn't from once being under such high pressure from the atmosphere it would have once had.

I'm sure there are other points I'm missing too.

[Trakar]

Surely Mercury's proximity to the Sun would make it a candidate for losing any atmospere to a severe coronal ejection as in a direct hit.
It wouldn't be a good day on Mercury.

Mercury at its current mass and with no magnetic field would have lost any atmosphere it had from simple solar wind erosion in fairly short order,...but there is a big difference between Mercury and the type of atmosphere it might have once possessed and a gas giant and the gravity constrained atmosphere it would possess.

[John Xenir]

4) I seem to remember that there where other properties that a core would have that mercury doesn't from once being under such high pressure from the atmosphere it would have once had.

What are these properties anyway? How would even look such planet?

[WayneFrancis]

What are these properties anyway? How would even look such planet?

I'd have to go find the thread. I don't know off the top of my head.

[Trakar]

I'd have to go find the thread. I don't know off the top of my head.

Diamonds everywhere!


metallic hydrogen permafrost?


some really dense granite.


A charred 2-20 Earth mass nugget of a planet
that dares mere asteroids to "Bring it on!"





maybe a parrot








or a nice shrub.

[korjik]

Mercury has a magnetic field.

[Trakar]

---

Mercury has a magnetic field.

It also has a core that makes up ~70% of the planet's mass (its almost the same mass as the Earth's core!). Which is one reason that Mercury is thought to have had much greater overall mass (in the Venus to Earth range) at some point in its early history. Given the strength of its magnetic field, however, it is probably accurate to refer to it as a mostly residual or remnant magnetic field.

http://www-ssc.igpp.ucla.edu/personn...pers/merc_mag/

[neilzero]

Can we have a gas not quite gas giant with 1/4 the mass of Neptune? Neil

[neilzero]

Last I heard, Jupiter had a core of metallic hydrogen at a temperature of 25,000 degrees k, and was hotter during all of the past 4.5 billion years. Do we think the metallic hydrogen floats on a smaller core of mostly iron? If present Jupiter lost half of it's present mass, due to loss of most of it's atmosphere, the metallic hydrogen would feel less pressure and revert to gas = plasma. Would iron vapor in Jupiter's atmosphere fall to the core to replace the metallic hydrogen? Would the core of Jupiter cool due to the change from metallic to gas hydrogen with lower density? if not, why do we think a remnant of Jupiter would have an iron core, instead of a something else core?
So Neptune has much less mass than Jupiter, no metallic hydrogen, but almost as hot a core. Do we have any good theories on the present composition of the core of Neptune? Is it likely mostly iron, like Mercury? Are we even reasonably sure that Mercury has a mostly iron core? About 20% of the elements have a higher density than iron. Neil